The invention relates to a rotary pressure exchanger for transfer of pressure energy from one fluid to another by direct contact positive displacement. Such pressure exchangers are designed with a rotor being rotated by the fluid streams or by an external motor. The flow driven designs use either a sleeve or a central axle for positioning of the rotor between two pairs of end covers. The axle based pressure exchanger has the distinct advantage of being a very compact design as the rotor can be much larger within same size housing and accommodate up to twice the flow. Rotary pressure exchangers have become a leading energy recovery device for Sea Water Reverse Osmosis (SWRO) due to exceptional high efficiency and small footprint. Seawater is very corrosive with low viscosity leading to many challenges that demands the ultimate performance of materials, design and manufacturing.
U.S. Pat. No. 7,306,437 discloses a pressure exchanger of having a flow driven rotor positioned on an axle. In order to prevent axial lock-up were the rotor is forced into mechanical contact in the peripheral areas, the end covers have a central step where the end clearance with the rotor is reduced. This feature is difficult to manufacture as it prevents flatness to be addressed by precision lapping and also lacks any true axial bearing stiffness as it relies only on a weak hydrodynamic effect.
U.S. Pat. No. 5,988,993 discloses a rotor positioning system for a sleeve based rotary pressure exchanger using a known element of a step bearing on the rotor surface. A single central manifold receives high pressure fluid that drains over the step bearings to each end of the rotor and into a collector manifold located in the end covers, from where fluid flows into the low pressure stream. This arrangement provides basically only hydrostatic radial bearing stiffness as only the limited and narrow small clearance area at the end of each step bearing is subject to hydrodynamic action.
Thus, there is a need for a rotor positioning system for rotary pressure exchangers that does not have the above noted disadvantages of existing flow driven devices.